I see where you're coming from, but this to me seems more like a property of the fediverse than an issue with Kbin in particular. Right now, anyone can make an instance and choose to show downvotes. Someone could make their own instance, gather downvotes federated from other instances, and make a list of who's downvoted who. Being on an instance that doesn't show downvotes doesn't hide your downvotes from everyone — just you and others on instances that don't show them.
On this end, I feel that Kbin instances are just being transparent about the publicity of your votes. If anyone can see your downvotes just by looking at an instance that shows them, I think it's important that people are aware of that. Showing public votes is sort of telling you, "Hey, people can see how you voted on the fediverse," and that's preferable to pretending that nobody can see them.